Nowadays, the major energy source for humankind comes from petroleum. The power or electricity required to drive a car or run a thermal power plant is supplied by burning petroleum. However, the heat and exhaust generated during the combustion of the petroleum not only can deteriorate the air quality, but can worsen the global warming effect. Besides, the yield of the petroleum will reach culmination in ten years and then will decline year by year. This means that the oil price (including the electricity tariff) will not be cheap anymore. Therefore, the energy crisis might come up eventually and cause global economic storm.
In view of the forthcoming global economic storm, renewable energy has been discovered to provide electricity or mechanical power efficiently and economically for households or industries. Thus far, the development of renewable energy has become an important energy policy for developed countries as a win-win strategy for power generation and environmental protection. Among various renewable energy, such as solar energy, wind energy, tidal energy, geothermal energy, and biowaste energy, the solar energy has become the mainstream as the solar energy generator has the advantages of high eco-friendliness, easiness of installation, matureness of commercial merchandising, and the overwhelming promotion lead by the country. Hence, solar energy has become a major choice for developed countries in pursuing distributed power supply system.
Referring to FIG. 1, in which the circuitry of an inverter according to the prior art is shown. As shown in FIG. 1, the inverter 1 is used in a solar grid-connected photovoltaic system, and thus the inverter 1 is also known as a photovoltaic inverter, or PV inverter. The inverter 1 is configured in a non-isolated and full-bridge topology, and includes an input filter 10, a full-bridge switch circuit 11, and an output filter 12. The input filter 10 is consisted of a first capacitor C1 that receives a DC input voltage VDC generated by a solar cell and filters the DC input voltage VDC. The full-bridge switch circuit 11 is connected to the output filter 12 and is consisted of switch elements S1-S4, in which the first switch element S1 is connected in series with the second switch element S2 and the third switch element S3 is connected in series with the fourth switch element S4 so as to form a full-bridge circuit with two bridge arms. The switch elements S1-S4 are controlled by a control unit (not shown) to turn on or off, thereby allowing the full-bridge switch circuit 11 to convert the filtered DC input voltage VDC into an AC modulating voltage VT. The output filter 12 is connected to the full-bridge switch circuit 11 and is consisted of a first inductor L1, a second inductor L2, and a second filtering capacitor C2. The output filter 12 is used to remove the high-frequency components of the AC modulating voltage VT to output an AC output voltage Vo to a grid G.
Generally, the switch elements S1-S4 of the full-bridge switch circuit 11 are configured to operate with the pulse-width modulation (PWM) fashion. More precisely, the switch elements S1-S4 of the full-bridge switch circuit 11 can operate under the bipolar switching mode or the unipolar switching mode. As the full-bridge switch circuit 11 is operating under the unipolar switching mode, only one bridge arm consisted of two switch elements are configured to conduct high-frequency switching operations, the AC modulating voltage VT is fluctuating between 0 and the positive DC input voltage VDC or fluctuating between 0 and the negative DC input voltage −VDC. On the contrary, as the full-bridge switch circuit 11 is operating under the bipolar switching mode, the switch elements S1-S4 are configured to conduct high-frequency switching operations, the AC modulating voltage VT is fluctuating between the positive DC input voltage VDC and the negative DC input voltage −VDC. Therefore, the switching loss of the full-bridge switch circuit 11 operating under the unipolar switching mode is less than the switching loss of the full-bridge switch circuit 11 operating under the bipolar switching mode. In other words, the full-bridge switch circuit 11 will have better conversion efficiency under the unipolar switching mode. However, as a parasite capacitance Cp is existed between the solar cell which generates the DC input voltage VDC and the ground terminal, as shown in FIG. 1, the modulating voltage VT will have high-frequency components when the full-bridge switch circuit 11 is operating under the unipolar switching mode. Thus, the relative voltage drop between the positive output terminal of the switch circuit 11 and a specific node within the inverter 1 and the relative voltage drop between the negative output terminal of the switch circuit 11 and the specific node within the inverter 1 can not be set to maintain their total average value at any switching point at a constant value. This would result in a significant voltage drop across the parasite capacitance Cp and cause leak current, thereby endangering human users and equipment.
Referring to FIG. 2, in which a different circuitry of the inverter according to the prior art is shown. In FIG. 2, the inverter 2 is configured as a neutral point clamped inverter (NPC inverter). The inverter 2 includes an input filter 20, a switch circuit 21, and an output filter 22. The connecting relationship between the input filter 20 and the output filter 22 are similar to the connecting relationship between the input filter 10 and the output filter 12 of FIG. 1. Thus, the details in connection with the input filter 20 and the output filter 22 will not be given herein. The switch circuit 21 is consisted of switch elements S1-S12, in which the switch elements S1-S6 form a first switch branch and switch elements S7-S12 form a second switch branch.
When the inverter 2 is applied to a solar grid-connected photovoltaic system, the parasite capacitance Cp within the solar cell will not undergo a significant voltage drop due to the switching operations of the switch elements S1-S12. Thus, the leak current can be avoided. However, as the inverter 2 includes twelve switch elements, the manufacturing cost of the inverter 2 is very high. More disadvantageously, the switching loss of the inverter 2 will be aggravated as the inverter 2 uses twelve switch elements to carry out switching operations. This would deteriorate the conversion efficiency of the inverter 2.
Thus, the applicants endeavor to develop an inverter with a better conversion efficiency and lower leak current.